Method for determining the signal-to-noise ratio of an optical signal

ABSTRACT

A method for determining signal-to-noise ratios and noise levels in an optical signal is disclosed, the first polarisation state of which is converted into a second polarisation state by means of number of tunings of a polarisation regulator. Defined changes to the second polarisation state are adjusted on the Poincare sphere by means of the polarisation regulator, whereby power values for the optical signal are determined after selection of a component of the electrical field. Some of the determined power values for the optical signal are stored and serve for the calculation of the signal-to-noise ratio of optical signals. Said method is rapid, requires little complicated equipment and is particularly suitable for a WDM transmission system in which many channels in a WDM signal are transmitted with small channel separations.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/DE2003/002671, filed Aug. 8, 2003 and claims the benefitthereof. The International Application claims the benefits of GermanPatent application No. 10239305.2 DE filed Aug. 27, 2002, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method and a device for determining thesignal-to-noise ratio (OSNR) of an optical signal according to thepreambles of the claims.

BACKGROUND OF THE INVENTION

The multichannel WDM signal transmission range that can be spanned usingwave division multiplex (WDM) transmission systems is limited, amongother things, by the amplified spontaneous emission (ASE) produced inoptical amplifiers as noise power which is superimposed on the opticalsignals in the channels. This noise power must be measured for optimumadjustment of the transmission characteristics.

Normally the noise power ASE occurring at a certain wavelength spacingfrom a channel is measured at smaller and greater wavelengths and thenoise power ASE superimposed on the channel is calculated byinterpolation. Because of the substantial increase in the number ofwavelength channels and the accompanying reduction in channel spacing,this method can no longer be used. In addition, the components used forinfluencing the spectrum and for coupling signals in and out in moderntransmission systems within the route preclude the use of this method.In such transmission systems therefore a method must be used whichallows the noise power ASE superimposed on the channels to be measureddirectly.

To this end a method known as polarization nulling has been proposedwhich makes use of the fact that the signal portion resulting from thenoise component ASE is not polarized. However, the main disadvantage ofall hitherto known proposals for implementing this method is that eachchannel has to be individually selected by spectral filtering and adefined polarization state for optimum suppression of the polarizedsignal portion must be set using a polarization controller. This methodis therefore very complex/costly and results in long measurement times.The two following articles describe the basic principles of the method:“OSNR Monitoring Technique Based on Polarisation Nulling Method”, J. H.Lee, D. K. Jung, C. H. Kim, Y. C. Chung, WEEE Photonics TechnologyLetters, Vol. 13, No. 1, January 2001; “Improved OSNR MonitoringTechnique Based on Polarisation Nulling Method”, J. H. Lee, Y. C. Chung,Electronics Letters, 19^(th) Jul. 2001, Vol. 37, No. 15.

DE 10049769 A1 likewise describes a device and a method for measuringthe optical signal-to-noise ratio (OSNR), utilizing the fact that thesignal portion, unlike the noise portion, is linearly polarized. After avariable optical bandpass filter (VOBPF) the previously amplified inputsignal is divided into four subcomponents and the Stokes parameters aredetermined. A computing equipment calculates both the power of thepolarized input signal and the noise power. The ratio of the twoprovides the OSNR. The device measures the OSNR for the entire spectralrange by sequentially varying the passing wavelength of the VOBPF,started with the smaller wavelengths, and determining the peak value forthe signal power from the measured power values. Also with this methodthe equipment complexity is high due to the necessary computing andevaluation unit combined with the filter unit.

In “Optical Signal-To-Noise Ratio Measurement In WDM Networks UsingPolarization Extinction”, M. Rasztovits-Wiech et al., ECOC 98, 20-24September, Madrid, p. 549-550 an arrangement for measuring thesignal-to-noise ratio is presented in which a WDM signal is injectedinto a polarization controller, then into a linear polarizer andsubsequently into an optical spectrum analyzer or a power measurementdevice with preceding tunable optical filter. The tunable filter is setin such a way that the power of an individual channel is completelytransmitted and the remaining portion of the WDM spectrum is suppressed.The polarization controller is adjusted until the power meter indicatesa minimum signal. The polarizer is then brought into the orthogonalposition so that the power measurement device indicates a maximum value.The difference between the maximum signal and the minimum signalincreased by 3 dB provides the signal-to-noise ratio OSNR referred tothe bandwidth of the tunable filter. One disadvantage of this method isthe large amount of time required for measuring a very large number ofWDM channels, as all the channels have to be sequentially measuredindependently as described above.

Another method consists of covering all polarization states on thePoincare sphere using a polarization scrambler and, for eachpolarization state set, recording an associated spectrum with the aid ofan optical spectrum analyzer. The minimum and maximum power determinedfrom analysis of all the recorded spectra is then used for calculatingthe signal-to-noise ratio OSNR. The minimum power occurs precisely whenthe signal is completely suppressed by the polarizer, whereas in thecase of maximum power the signal power plus the noise power ASE ismeasured.

U.S. 2001/0052981 A1 discloses a method for measuring thesignal-to-noise ratio of an optical signal which constitutes a standardpolarization nulling procedure, wherein the rotation angles between alambda/4 plate and a polarizer are set as manipulated variables by meansof a closed-loop control system. A major disadvantage is that aparticular polarization state must first be set at the polarizer input.After the polarizer is adjusted, the minimum and maximum of the opticalsignal are determined from the measurement results. As a control systemfor 360° rotation of the polarizer is necessary for measuring thesignal-to-noise ratios or to achieve one or two required polarizationstates, this method exhibits a disadvantageous measurement redundancy,making it a time-consuming process.

In practice it is of course impossible to cover all polarization states.A more or less large measurement error remains depending on the numberof states selected and the speed at which the polarization state of achannel changes in the transmission system.

SUMMARY OF THE INVENTION

The object of the invention is to specify a method and a device withwhich the signal-to-noise ratio of the signals of an optical signal canbe determined with minimal complexity and as quickly as possible on thebasis of polarization nulling. The method should provide particularadvantages for analyzing optical wavelength division multiplex (WDM)signals.

This object is achieved in respect of its method aspect by a methodhaving the features set out in the claims and in respect of its deviceaspect by a device having the features set out in the claims.

The determined amplitude values of the optical signal are inventivelystored on the basis of a method for determining the opticalsignal-to-noise ratio OSNR of an optical signal having a firstpolarization state which is converted by means of a plurality ofsettings of a polarization controller into a second polarization state,whereby defined changes in said second polarization state, for whichamplitude values of the optical signal are determined, are set on thePoincare sphere by the polarization controller. The signal-to-noiseratio OSNR of the optical signal or of another optical signal isdetermined from a calculated value of the stored amplitude values.

According to the invention, the signal-to-noise ratios OSNR of one ormore channels are determined by means of interpolation on the basis of alimited number of stored amplitude values. This is achieved bydetermining the calculated value as an interpolated deviation of thestored amplitude values squared.

A significant advantage of the method according to the invention is thatinstead of discrete, channel-specific, fine and slow settings oradjustments of the polarization controller, only a few pre-settings fordetermining amplitude values to be stored are necessary in the case ofdefined polarization states. This therefore constitutes a very fastmethod for determining other signal-to-noise ratios OSNR.

A further advantage of the invention is that it is not necessary to seta particular polarization state selectively, so that no complexadjustment is necessary.

As measurements are performed for any polarization states, a pluralityof measurement points for all the channels is simultaneously obtainedfor a given setting of the two plates, so that the measurement time isindependent of the number of channels.

Advantageous developments of the invention are set out in thesub-claims.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the invention will now be explained infurther detail with reference to the accompanying drawings in which:

FIG. 1: shows a device for performing the method according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

To provide a simpler illustration of the method according to theinvention, a device according to FIG. 1 is selected in such a way that aWDM signal S is first fed to a polarization controller PS comprising aλ/4 plate E1 and a λ/2 plate E2 as phase retarder plates. Thepolarization controller PS is followed by a polarizer POL. For differentsettings of the polarizer or of the polarization state allowed throughfrom the polarization controller, the spectral power density at theoutput of this device is recorded in each case by means of an opticalspectrum analyzer OSA. The optical spectrum analyzer OSA can be precededby a wavelength demultiplexer or a wavelength-selective filter, so thatselected channels or only one channel of the WDM signal can be recorded.However, demultiplexing is in practice unnecessary. Connected to theoptical spectrum analyzer OSA is an optical signal-to-noise ratio (OSNR)determination unit EE in which an interpolation and a deviation searchof the amplitude values recorded at the optical spectrum analyzer OSAare performed for determining the measured signal-to-noise ratio OSNRaccording to the invention. The determination unit EE controls arotating device DV for the plates E1, E2. Connected to the spectrumanalyzer OSA or the determination unit EE is a memory unit SP fortabulating the signal amplitude values measured at the optical spectrumanalyzer OSA for different settings of the phase retarder plates E1, E2.

An electrical field vector {right arrow over (E)} of a plane wave withfrequency ω and wave number k traveling in z-direction in an orthogonalcoordinate system with x-, y- and z-axes is described mathematically bythe expression: $\overset{->}{E} = {\begin{pmatrix}{E_{x}{\mathbb{e}}^{{\mathbb{i}\varphi}_{x}}} \\{E_{y}{\mathbb{e}}^{{\mathbb{i}\varphi}_{y}}}\end{pmatrix}{\mathbb{e}}^{{\mathbb{i}}{({{\omega\quad t} - {kz}})}}}$

-   -   where E_(x), φ_(x) and E_(y), φ_(y) are the amplitude and phase        of the components of the electrical field vector {right arrow        over (E)} in the x- and y-direction respectively. Normalizing to        E=√{square root over (E _(x) ² +E _(y) ² )}        produces the so-called Jones vector {right arrow over (J)}:        ${\overset{->}{J} = {\frac{1}{E}\begin{pmatrix}        {E_{x}{\mathbb{e}}^{{\mathbb{i}\varphi}_{x}}} \\        {E_{y}{\mathbb{e}}^{{\mathbb{i}\varphi}_{y}}}        \end{pmatrix}}},$        which describes the polarization state of the wave.

Only the difference Δφ=φ_(y)−φ_(x) is of importance for the polarizationstate, so that the phase of a component may be set to zero. With φ_(x)=0we get: $\overset{->}{J} = {\frac{1}{E}{\begin{pmatrix}E_{x} \\{E_{y}{\mathbb{e}}^{\mathbb{i}\Delta\varphi}}\end{pmatrix}.}}$

The effect of optical components on the polarization of a plane wave canbe described by Jones matrices which transform the Jones vectors in theform of a linear map. Matrix representations are always linked to theselection of a specific base. This means that when specifying a matrixthe position of the coordinate axes is fixed. In this embodiment thex-component of the incoming wave to the linear polarizer POL is subjectto maximum transmission and the y-component of this wave is completelysuppressed.

The Jones matrix of the λ/4 plate whose fast axis forms the angle δ withthe x-axis can be represented as follows:$M_{\lambda/4} = {\frac{1}{\sqrt{2}}{\begin{pmatrix}{1 + {{{\mathbb{i}} \cdot \cos}\quad 2\delta}} & {{{\mathbb{i}} \cdot \sin}\quad 2\delta} \\{\sin\quad 2\delta} & {1 - {{{\mathbb{i}} \cdot \cos}\quad 2\delta}}\end{pmatrix}.}}$

The Jones matrix of the λ/2 plate is of the form:${M_{\lambda/2} = {{\mathbb{i}}\begin{pmatrix}{\cos\quad 2\theta} & {\sin\quad 2\theta} \\{\sin\quad 2\theta} & {{- \cos}\quad 2\theta}\end{pmatrix}}},$where θ denotes the angle between the fast axis of this plate and thex-axis.

The device shown in FIG. 1 will now be considered in the light of thistheory. The arrangement comprising the λ/4 plate and the λ/2 plate isdescribed by the following matrix wherein the elements in the second roware intentionally not shown, as they only affect the y-component of theelectrical field {right arrow over (E)} suppressed by the polarizer POL:$M = {{M_{\lambda/2} \cdot M_{\lambda/4}} = {\frac{\mathbb{i}}{\sqrt{2}}\begin{pmatrix}{{\cos\quad 2\theta} + {{\mathbb{i}} \cdot {\cos\left( {{2\theta} - {2\delta}} \right)}}} & {{\sin\quad 2\theta} - {{\mathbb{i}} \cdot {\sin\left( {{2\theta} - {2\delta}} \right)}}} \\\ldots & \ldots\end{pmatrix}}}$

For the signal power I=|{right arrow over (E)}|² measured at the opticalspectrum analyzer OSA and therefore I=|M·{right arrow over (J)}51 ² weobtain:$I = {\frac{1}{2}\left\lbrack {{E_{x}^{2} \cdot \left( {{\cos^{2}2\theta} + {\cos^{2}\left( {{2\theta} - {2\delta}} \right)}} \right)} + {E_{y}^{2} \cdot \left( {{\sin^{2}2\theta} + {\sin^{2}\left( {{2\theta} - {2\delta}} \right)}} \right)} + {2E_{x}{E_{y} \cdot \cos}\quad{{\Delta\varphi} \cdot \left( {{\sin\quad 2{\theta \cdot \cos}\quad 2\theta} - {{\sin\left( {{2\theta} - {2\delta}} \right)} \cdot {\cos\left( {{2\theta} - {2\delta}} \right)}}} \right)}}} \right\rbrack}$where Δφ=φ_(y)−φ_(x) is as defined above.

In normalized form this yields:$\left. {\frac{I}{E_{x}^{2} + E_{y}^{2}} = {\frac{1}{2} + {{\cos\left( {{4\theta} - {2\delta}} \right)} \cdot \left\lbrack {{{\left( {q^{2} - {1/2}} \right) \cdot \cos}\quad 2\delta} + {{q \cdot \sqrt{1 - q^{2}} \cdot \cos}\quad{{\Delta\varphi} \cdot \cos}\quad 2\delta}} \right)}}} \right\rbrack + {{{\sin\left( {{4\theta} - {2\delta}} \right)} \cdot q \cdot \sqrt{1 - q^{2}} \cdot \sin}\quad{\Delta\varphi}}$where q denotes the distribution of the total power to the twocomponents E_(x), E_(y) at the input of the measurement device accordingto the following equations:$E_{x} = {\frac{q}{\sqrt{E_{x}^{2} + E_{y}^{2}}}\quad{and}}$$E_{y} = {\frac{\sqrt{1 - q^{2}}}{\sqrt{E_{x}^{2} + E_{y}^{2}}} \cdot {\mathbb{e}}^{{\mathbb{i}}\quad{\Delta\varphi}}}$

This representation indicates that the dependence of the intensity I onthe angle θ can be described by a sinusoidal function sin(4θ−2δ+ρ) (ρrepresenting a phase which, however, is irrelevant to the presentinvention).

The square A² of the deviation of this sinusoidal curve—i.e. twice theamplitude—can be calculated as:$\left. {A^{2} = {{4 \cdot \left\lbrack \left\{ {{{\left( {q^{2} - {1/2}} \right) \cdot \cos}\quad 2\delta} + {{q \cdot \sqrt{1 - q^{2}} \cdot \cos}\quad{{\Delta\varphi} \cdot \cos}\quad 2\delta}} \right) \right\}^{2}} + \left\{ {{q \cdot \sqrt{1 - q^{2}} \cdot \sin}\quad{\Delta\varphi}} \right\}^{2}}} \right\rbrack$or$A^{2} = {4 \cdot \left\lbrack {{\frac{1}{2}\left\{ {\left( {q^{2} - {1/2}} \right)^{2} + {q^{2} \cdot \left( {1 - q^{2}} \right) \cdot \left( {1 + {\sin^{2}{\Delta\varphi}}} \right)}} \right\}} + {\frac{1}{2}{\left\{ {\left( {q^{2} - {1/2}} \right)^{2} - {{q^{2} \cdot \left( {1 - q^{2}} \right) \cdot \cos^{2}}{\Delta\varphi}}} \right\} \cdot \cos}\quad 4\delta} + {{\left( {q^{2} - {1/2}} \right) \cdot q \cdot \sqrt{1 - q^{2}} \cdot \cos}\quad{{\Delta\varphi} \cdot \sin}\quad 4\delta}} \right\rbrack}$

This variable in turn shows a sinusoidal dependence on the angle δ. Forthe method shown it is significant that the maximum of thisvariable—irrespective of q and Δφ—is always 1 and therefore gives thesignal power.

In short, the invention is based on the knowledge that the power Itransmitted by the polarizer POL and measured can be described as asimple trigonometric function dependent on the two setting angles θ andδ of the λ/2 plate and λ/4 plate respectively.

The measured power I at the optical spectrum analyzer OSA is stored fora number of defined settings of the plates E1 and E2 e.g. in atwo-dimensional table as a function of the manipulated variables δ andθ. The individual process steps will now be described in detail. Tosimplify the description, the method will first be discussed for asingle channel. It will then be explained how the signal-to-noise ratioOSNR of all channels can be determined simultaneously, e.g. in a WDMsystem. This method is preferably suitable for any optical multiplexsignals prior to demultiplexing.

In the case of a fixed setting of the □/4 plate E1 e.g. at an angle δ1,the power of the channel after the polarizer POL is recorded for n (n=1,2, . . . N) different settings i.e. for n angles θ1, θ2, . . . , θN ofthe □/2 plate E2 as a set or spectrum S_(δ1) of power values.

For any permanently selected position of the □/4 plate E1 at otherangles δ2, . . . , δM (m=2, . . . (M−1)) and time-constant polarizationof the incident light wave, there is sinusoidal dependence between themeasured power I after the polarizer POL and the angle θ of the fastaxis of the □/2 plate E2 with respect to the polarizer POL. The maximumand the minimum of this curve are dependent on the position of the □/4plate E1 and will now be denoted as I_(max) and I_(min) respectively.

The powers I_(max) and I_(min) are determined from the measurements fora plurality of positions of the □/2 plate E2 by means of a suitablecurve fit to the sine curve and stored, a corresponding deviation A₁from the powers I_(max) and I_(min) also being stored.

Steps (1) to (3) are now repeated for various positions of the □/4 plateE1 (number m, m>1). M values for I_(max) and I_(min) are thereforedetermined and stored, further corresponding deviations A₂, A₃, . . . ,A_(M) from the powers I_(max) and I_(min) also being stored.

If the square of the difference I_(max)−I_(min) is now plotted above theangle δ for the m positions of the □/4 plate, the maximum value for(I_(max)−I_(min))² can be determined by a suitable fit to the sinusoidalcurve.

The resulting maximum corresponds to the signal power. As the sum of thesignal power and noise power is known from a power

-   -   measurement at the input of the device, the noise power and        therefore also the signal-to-noise ratio OSNR can be determined        by subtraction.

The procedure for a multichannel WDM signal is now obvious. Instead ofthe power of just a single channel, a power spectrum S1, S2, . . . isrecorded for each combination of settings of the two birefringent platesE1, E2 so that the powers of all the channels after the polarizer POLare determined in each case. The evaluation by interpolation of thesinusoidal curves can now be performed separately for each channel asbefore.

1-9. (canceled)
 10. A method for determining the signal-to-noise ratioof arbitrarily polarized optical signals of different wavelength thatare combined to form a wave division multiplex signal according to apolarization nulling method, comprising: recording and storing powerspectra of the wave division multiplex signal for a first definedsetting m=1 (m=1, 2, . . . M) of a first polarization-optical phasecontroller and for N (n=1, 2, . . . N) settings of a secondpolarization-optical phase controller; determining and storing a maximumdeviation for the optical signals from the power spectra; recording andstoring the power spectra of the wave division multiplex signal for(M−1) new settings of the first polarization-optical phase controllerand for N settings in each case of the second polarization-optical phasecontroller; determining and storing from the stored power spectra foreach setting of the first phase controller the maximum deviations withm=1, 2 . . . (M−1) of the signals; and calculating the signal-to-noiseratio for the optical signals based on all of the deviations.
 11. Themethod according to claim 10, wherein the deviation of an optical signalis determined by an interpolation.
 12. The method according to claim 10,wherein the signal power of the optical signal is determined by aninterpolation of the squared deviations.
 13. The method according toclaim 10, wherein a sum of the signal and noise power is determined bymeasuring the power at the input of a polarization controller and anoise power is determined by subtracting a determined signal power ofthe optical signal.
 14. The method according to claim 10, wherein thenumber of polarization controller settings is selected on a minimumbasis depending on a specified relationship between precisiondetermination of the signal-to-noise ratio and measurement time.
 15. Themethod according to claim 10, wherein phase shifts between thecomponents of an electrical field vector of an optical signal and apolarizer are performed by phase retarder plates as polarization-opticalphase controllers.
 16. The method according to claim 10, wherein a firstphase retarder plate can be set using a first rotation angle and asecond phase retarder plate can be set using a second rotation angle.17. The method according to claim 16, wherein the settings of the firstand second phase retarder plates and are implemented in such a way thata first phase shift is set for a first rotation angle and a plurality Nof angles are set from which a set of N power values is recorded, fromthese power values a first sinusoidal interpolation curve is determinedwhose deviation is stored in a table, the settings of the angles arerepeated for further rotation angles with m>1 for recording furtherpower values from which further deviations are stored and whose valuesare squared and interpolated with a sinusoidal curve as a function, andthe signal power of the optical signal is determined from the deviationof the sinusoidal curve by the signal-to-noise ratio (OSNR) is derivedfor the optical signals.
 18. The method according to claim 10, wherein aresolution cell with a bandwidth equal to or less than the spectralwidth of a channel of a WDM signal is selected to record the powervalues of an optical signal.
 19. A device for determining thesignal-to-noise ratio of arbitrarily polarized optical signals ofdifferent wavelength which are combined to form a WDM signal accordingto a polarization nulling method, comprising: a memory unit added to anoptical spectrum analyzer for tabulating the power values of the spectrameasured at the optical spectrum analyzer for different settings of thephase controllers; and a determination unit connected to the opticalspectrum analyzer for calculating the signal-to-noise ratio byinterpolation and deviation searching of the power values recorded atthe optical spectrum analyzer, wherein after passing through a first anda second polarization-optical phase controller the optical signal isinjected into a linear polarizer with following optical spectrumanalyzer.